<!doctype html>
<canvas id='c'></canvas>

<script>
const canvas = document.getElementById('c'),
ctx = c.getContext('2d', {alpha: false});

canvas.width = 128;
canvas.height = 128;

ctx.fillStyle = 'green';
ctx.fillRect( 0, 0, 128, 128 );
ctx.fillStyle = 'blue';
ctx.fillRect( 0, 0, 64, 64 );
ctx.fillRect( 64, 64, 64, 64 );
</script>